The present invention generally relates to a technology for further improving the corrosion resistance inherent in stainless steel, more particularly to clad stainless steel remarkably improved in corrosion resistance and a cladding material for use in forming a cladding layer on the surface of stainless steel by the irradiation of a laser to improve the corrosion resistance. The invention also relates to a method of further improving the corrosion resistance of stainless steel, a device for forming a coating material film of uniform thickness on the inner surface of a pipe for the purpose of providing a coating layer of corrosion resistance or the like thereon, and a torch for irradiation of laser beams which is particularly suitable for irradiating a laser to the inner surface of a pipe.
In general, stainless steel is widely used in buildings, bridges, atomic plants, chemical plants and so on. The stainless steel inherently possesses good corrosion resistance, because, in the atmosphere, chromium in its structure combines with oxygen of the air to form a thin passive coating of chrome oxide on the surface of the stainless steel. However, in case the stainless steel is brought into contact with a corrosive fluid, such as hydrogen sulfide, chlorine ion and a hot water containing oxygen, pitting corrosion or cracks due to corrosion are liable to occur in the surface of the stainless steel.
Hitherto, when the potentiality of corrosion in the surface of the stainless steel is of concern, it has been considered to cope with this problem by using an acid or the like so as to strengthen its passive coating.
Even if a passive coating on the surface of the stainless steel is strengthened by acids or the like, however, it is impossible to prevent a corrosion phenomenon from gradually progressing unless a condition, in which the stainless steel is kept in contact with a corrosive fluid, is removed.
In view of the above, the present inventors have been thought of forming a cladding layer on the surface of the stainless steel with the use of a laser. This formation is particularly useful as a coating for the inner surface of a pipe to improve the corrosion resistance of pipes and the like in facilities for a nuclear reactor.
Meanwhile, hitherto, as a method of forming a film of a coating material for the purpose of providing a corrosion resisting coating layer on the surface of a metal, or for another purpose, brush coating, spraying or the like has been used. With these methods, although a coating material film of uniform thickness can readily be formed when a surface to be treated is exposed to the outside, it is difficult to form the same on the surface hiding inside, such as the inner face of a pipe. Particularly, it is impossible to partially form a coating material film at a deep position within the pipe. For the formation of such a cladding layer as described above, however, uniformly application of a coating material on the surface of stainless steel is necessary even when the objective to be treated is in a pipe shape. Thus, such a device, which is capable of readily forming a coating material of uniform thickness on the inner surface of a pipe, is desired.
Further, a laser has been applied to perforating in a plate, cutting and so forth, and various types of laser irradiation devices for these ends have been provided. However, only few devices among them are applicable to objectives which have surfaces of peculiar shapes to be treated. In particular, hitherto, there has been provided no device which can readily irradiate a laser to the inner surface of a pipe of a small diameter. The reasons for this are the technical difficulty that a device, or a torch, is disposed close to a portion to be treated where the material vigorously splashes and which reaches a high temperature, and little need of such a working. For the formation of a cladding layer described above, however, a device for readily irradiating a laser to the inner surface of a pipe with good operation efficiency is required. Thus, a torch capable of overcoming the above technical difficulty is desired.